This invention relates to key telephone systems and more particularly to an exclusion system for such telephone systems.
Prior art exclusion systems employ circuitry that either senses the potential on the subset's "A" lead or senses voltage across the subset's tip conductor and ring conductor. These signals are then amplified or used directly to close the subset's tip conductor and ring conductor common circuit.
Prior art exclusion systems provide privacy for the central office/private branch exchange (CO/PBX) key lines only. These prior art exclusion systems have not been designed to provide privacy on common path or party line paging and intercom systems. Exclusion systems that sense the voltage between the tip conductor and ring conductor are unreliable in as much as variation in cable lengths can cause intermittent operation. Other exclusion systems frequently contain large electromechanical relays which are costly and prone to contact contamination, reduced life expectancy, and slow operating speed. These exclusion systems also employ discrete circuitry, and when combined with the electromechanical relays, substantial amounts of operating current are required, often in the order of 35 ma (milliamperes) to 50 ma per unit.